The present invention relates to a solid-state imaging device such as a CMOS image sensor or a CCD image sensor including a photoelectric conversion unit in which a plurality of pixels is arranged, and, more particularly, to a back irradiation type solid-state imaging device in which a signal circuit is formed on one surface of a substrate so as to receive light from the other surface of the substrate, a method of manufacturing the solid-state imaging device, and an electronic apparatus using the solid-state imaging device.
In recent CCD type or CMOS type image sensors, the number of photons incident to a unit pixel is reduced with reduction of a pixel size. Therefore, sensitivity or an S/N ratio deteriorates. In the case where a Bayer array using a primary-color color filter is used in order to realize a pixel array in which currently widely-used red, green and blue pixels are arranged in a planar shape, in a red pixel, green and blue light are not transmitted by the color filter. Therefore, in the red pixel, since green and blue lights are used for photoelectric conversion, loss occurs in light use efficiency. In such a pixel configuration, false colors may be generated by performing an inter-pixel interpolation process so as to form a color signal.
In order to solve such problems, in Japanese Unexamined Patent Application Publication No. 2002-151673, a solid-state imaging device in which photoelectric conversion regions for photoelectrically converting green, blue and red lights with respective wavelengths are laminated in a vertical direction of the same pixel and a green photoelectric conversion region is formed of an organic photoelectric conversion film is suggested.
If the structure of Japanese Unexamined Patent Application Publication No. 2002-151673 is used, light loss does not occur in the color filter. In addition, since the interpolation process is not performed, false colors are not generated.
However, in the case where charges photoelectrically converted by the organic photoelectric conversion film are stored in an Si substrate, ohmic contact is necessary in the Si substrate in order to transmit a signal from the organic film to the Si substrate. In the case where electrons are used as photoelectrically converted carriers (signal charges), high-concentration N-type impurities are necessary in ion-implantation into the Si substrate to form an N-type diffusion layer. Thus, a PN junction having a high electric field is formed between the N-type diffusion layer and a peripheral P-well. Therefore, dark current due to the PN junction with the high electric field causes noise. In addition, if the photoelectrically converted signal charges are accumulated in the N-type diffusion layer in which contact is made from the organic photoelectric conversion film, the potential of the N-type diffusion layer is changed according to the accumulation of the signal charges and the electric field applied to the organic photoelectric conversion film is changed according to the accumulation of the signal charges. Then, linearity of an output with respect to an amount of light may not be obtained.
As a method of solving such a problem, in Japanese Unexamined Patent Application Publication No. 2003-31785, a configuration in which signal charges from an organic photoelectric conversion film are transferred to an N-type diffusion layer formed in an Si substrate and are then overflowed is suggested. The method will be described using FIGS. 17 and 18A, 18B and 18C. FIG. 17 is a schematic cross-sectional view of a solid-state imaging device using an organic photoelectric conversion film of the related art. FIG. 18A shows a plan configuration of the main portions of FIG. 17, FIG. 18B shows a cross-sectional configuration corresponding to FIG. 18A, and FIG. 18C is a potential diagram along XVIIIC-XVIIIC in the cross-section of FIG. 18B.
As shown in FIG. 17, in the solid-state imaging device 120 of the related art, an organic photoelectric conversion film 112 interposed between an upper electrode 113 and a lower electrode 111 is laminated directly on a light incidence side of a semiconductor substrate 100. The organic photoelectric conversion film 112 is a film provided for photoelectrically converting a green light. The lower electrode 111 is connected to an n-type diffusion layer (N+) 108 formed by ion-implanting n-type impurities with high concentration in a surface of a well region 101 of the semiconductor substrate 100 through a contact portion 110. That is, signal charges obtained by the organic photoelectric conversion film 112 are transferred to the n-type diffusion layer 108.
In the structure of the related art, a charge accumulation region 107 which is an n-type semiconductor region is formed in a region contacting the n-type diffusion layer 108, and a p-type high-concentration impurity region 109 is formed on a front surface side of the charge accumulation region 107. In addition, a floating diffusion portion FD is formed in a region adjacent to the charge accumulation region 107, and a transfer gate electrode 106 is formed on the semiconductor substrate 100 between the charge accumulation region 107 and the floating diffusion portion FD with a gate insulating film interposed therebetween. The floating diffusion portion FD includes an n-type high-concentration impurity region.
In the structure of the related art shown in FIG. 17, signal charges obtained by the organic photoelectric conversion film 112 are transferred to the n-type diffusion layer 108, and signal charges transferred to the n-type diffusion layer 108 overflow into the charge accumulation region 107. In practice, in order to overflow signal charges in a horizontal direction, an overflow barrier which is a low-concentration p-type impurity region is formed between the n-type diffusion layer 108 and the charge accumulation region 107 so as to obtain the potential configuration shown in FIG. 18C. The potential of the n-type diffusion layer 108 is decided by the overflow barrier, and signal charges going beyond the overflow barrier overflow into the charge accumulation region 107.
In the structure in which the signal charges overflow in the horizontal direction as in the solid-state imaging device 120 of the related art shown in FIGS. 17 and 18A, 18B and 18C, since two capacitors are formed in a pixel, it is disadvantageous in view of reduction of the pixel size. Since the overflow of the horizontal direction is used, an overflow path from the n-type diffusion layer 108 to the charge accumulation region 107 is one-dimensional, a potential gradient of the horizontal direction is necessary for efficient overflow, and the degree of difficulty involved with the potential design is large.
A vertical type overflow path for overflowing signal charges from the n-type diffusion layer 108 connected to the organic photoelectric conversion film 112 by the contact portion 110 to a deep region of the semiconductor substrate 100 is also suggested. FIG. 19A shows a schematic cross-sectional configuration in the case where a vertical type overflow path is configured and FIG. 19B is an enlarged diagram of the main portions thereof. In FIGS. 19A and 19B, portions corresponding to those of FIGS. 18A, 18B and 18C are denoted by the same reference numerals and the description thereof will be omitted.
In the case where the vertical type overflow path is formed, a charge accumulation layer 117 is formed in the semiconductor substrate 100 below the n-type diffusion layer 108. In the case where the vertical type overflow path is formed, the overflow path is two-dimensionally formed so as to increase an area thereof. Thus, it is advantageous in view of overflow. In addition, since potential design in a depth direction is preferable, a potential gradient is given by ion implantation energy and thus processing difficulty is decreased. However, in practice, as shown in FIG. 19B, the n-type diffusion layer 108 is formed as deep as possible so as to cover the damage of the semiconductor substrate 100 and the contact portion 110. Therefore, the charge accumulation layer 117 is separated from the surface of the semiconductor substrate 100 in a depth direction. Accordingly, in the case where signal charges are transferred from the charge accumulation layer 117 to the floating diffusion portion FD through a transfer gate, the charge accumulation layer 117 is separated from the transfer gate. It is disadvantageous in view of transmission.